ABSTRACT: This paper describes open research questions related to the competition and market structure of financial exchanges and argues that only a combination of industrial organization and finance can satisfactorily attack these questions. Two examples are discussed to illustrate how the combination of these two approaches can significantly enrich the analysis: the “network externality puzzle”, which refers to the question of why trading for the same security is often split across trading venues, and the impact of the multi-sided character of financial exchanges on pricing and profitability.